1. Field of the Invention
The present invention relates generally to an appararus and process for producing film from polyolefin materials and, more particularly, pertains to a novel mandrel which, when incorporated into a blown film die, allows linear low-density polyethylene to be extruded through the die economically, producing a film with improved quality and increased bubble stability.
2. Discussion of the Prior Art
With the advent of the economical production of narrow molecular weight linear lower density polyethylene resins (LLDPE) and the improved properties obtained from this resin, there has been great interest in the extrusion art to successfully, and hence economically, form extruded films therefrom.
The present state of the art related to the production of linear low density polyethylene films (LLDPE) has produced numerous modifications to existing equipment used to produce low density polyethylene film (LDPE) or high density polyethylene film (HDPE).
High pressure low density polyethylene, of broad molecular weight distribution, can be considered "soft" in shear and "stiff" in extension. Ethylene hydrocarbon copolymers having a narrow molecular weight distribution such as (LLDPE) exhibit the opposite rheology. They are "stiff" in shear and "soft" in extension. The terms "soft" and "stiff", as used herein, refer to the relative magnitude of shear and extensional viscosity . In tubular blown film extrusion of LLDPE resins, this property is manifested by reduced melt strength in extension, resulting in a film bubble which is unable to resist deformation caused by the increased cooling required by increased throughput rates. An improved process for extruding film by tubular blown film extrusion of molten, linear ethylene hydrocarbon copolymers having a narrow molecular weight distribution providing improved bubble stability and enhanced film quality has been developed as hereinafter described.
Narrow molecular weight distribution, transition metal catalyzed, ethylene hydrocarbon copolymers have been extruded into film by conventional techniques such as blown film extrusion and slot cast extrusion.
In tubular blown film extrusion, the polymer melt is extruded annularly through a die to form a tube which is expanded to the desired extent, cooled or allowed to cool and flattened by passage through a collapsing frame and a set of nip rolls. A positive pressure of gas is maintained inside the tubular bubble to provide the desired expansion of the tubular film. As the polymer exits the annular die, the extrudate cools, its temperature falls below its melting point, and it solidifies. As the film is so cooled, crystallization occurs, changing the optical properties, and a frost line forms, at a point proximate the annular die by a distance controlled by the cooling rate.
In slot cast film extrusion, the molten polymer is extruded through a slot die and quenched, employing a chill roll or water bath. The optical properties of film so formed are much improved as compared to tubular blown film by virtue of the rapid extrudate cooling rate and resulting rapid crystallization to small spherulites. Higher temperatures may be employed reducing shear stress in the die and raising the threshold for melt fracture. Melt strength is also not a process limitation.
The extrusion of low pressure low density polyethylene affords special difficulties, in that the narrow weight distribution of such resins provides reduced shear thinning behavior at extrusion grade shear rates such that higher pressure and higher power is required during extrusion. Another consequence, as aforementioned, is the poor melt strength developed, requiring modifications in cooling equipment to maintain the stability of the film bubble in tubular extrusion.
When extruded through narrow gap dies the `linear low` resins generate very high extrusion head pressures. Shear stresses are high and the extrudate tends to melt fracture. Melt fracture refers to the phenomenon in which a resin extrudate becomes rough and nonuniform due to melt instabilities during polymer flow. When the polymer extrudate takes the form of a film, surface distortions, induced by melt fracture can be "frozen-in" as the extrudate cools and solidifies. These surface distortions can seriously detract from the mechanical strength of the film. These shear related problems also severely limit extruder output rate.
It has been found that when the die gap of the extrusion devices used in extruding films from these resins are greater than about 50 mils, extruder output rate can be increased significantly. Drawdown can become quite high. Head pressures, shear stresses in the die, and the tendency of the resin to melt fracture are all reduced. In essence, extensional deformation is substituted for shear deformation, accomodating the "stiff" shear and "soft" extensional rheology of these narrow molecular weight distribution linear ethylene hydrocarbon copolymers.
The problems associated with narrow molecular weight low density linear polyethylene resins is particularly acute when attempting to extrude film through existing apparatus which are configured to extrude low, medium and high density, non-linear polyethylene films. As the state of the present art exists, running LLDPE on unmodified LDPE equipment can result in anywhere from a 20-50% loss in lb/hr throughput. Though downgauging often offsets this drop by yielding comparable linear ft/hr, machinery and resin suppliers are hard at work designing LLDPE equipment that reportedly attains or exceeds LDPE output. It is, of course, possible but impractical and uneconomical to modify process parameters to respond to these disparate resin requirements.
Trying to force LLDPE through an LDPE blown film die raises two immediate problems: (1) system pressures rise because the higher viscosity resin puts up more resistance, and (2) `sharkskin` or `applesauce` (surface irregularities) can appear at shear rates that wouldn't normally fracture LDPE. One approach to offsetting higher pressures, is to employ larger die gaps as aforesaid. (However, in tubular blown film processes a wide die gap and low polymer melt strength leave a thick, easily distorted bubble below the frost line).
Computer designed spiral die bodies may be used to minimize the pressure build up exerted on the LLDPE as it passes through the die. See Running the Linear Lows, Plastics Technology, February 1981, p. 65-71 by Michael Hartung. Melt fracture may be minimized changing the geometry of the melt passageway, including shortening the land length to 0.25 to 0.50 inch, and providing a constrictor zone to reduce flow variations. For typical examples of the most recent art see U.S. Pat. Nos. 4,267,146; 4,282,177; 4,321,229, and 4,330,501.
U.S. Pat. No. 4,321,229 discloses an improved method for extruding linear polyolefin materials having high viscosities wherein a novel rotary extruder is provided having a feeding, metering and transition section.
U.S. Pat. No. 4,243,619 discloses a process for making film from low density ethylene hydrocarbon copolymer which comprises extruding the copolymer through a die having a die gap within the range of greater than 50 mils to approximately 120 mils, to provide a film having improved optical and mechanical properties.
U.S. Pat. Nos. 4,267,146 and 4,282,177 also refer to the use of a die gap greater than 50 mils together with a converging or diverging die section to avoid sharkskin melt fracture. It is taught that sharkskin melt fracture can be controlled or eliminated by the geometry at the exit of the die and is independent of die entrance or die land conditions.
U.S. Pat. No. 4,330,501 discloses an improved film bubble cooling technique for low strain hardening polymers such as LLDPE.
Additional related methods and apparatus are disclosed in the following materials.
U.S. Pat. No. 3,382,535 discloses an extrusion die that flares out in trumpet-like fashion towards the discharge orifice, the contours of which conform to a mathematical formula to enhance extrusion of the thermoplastic material without melt fracture. The plastic materials which are extruded are ordinarily sensitive to die taper angles. The mathematical formula is related to the critical shear rate of any selected plastic material, the minimum die radius and the die taper angle.
U.S. Pat. No. 3,914,366 describes a method and apparatus for forming a material in a thermoplastic extrusion die. During the formation of the material in the die, first and second areas of increased wall thickness are formed in the material, the thickened portions only are thereafter progressively decreased in thickness to form a uniform cross-section. The material is then discharged from the die.
U.S. Pat. No. 3,994,654 discloses a die for extruding a thermoplastic sheet having a controlled degree of microsurface roughness. The improvement comprises a beveled leading edge on at least one of the die lips when the bevel is away from the die orifice.
U.S. Pat. No. 4,267,146 describes a method for reducing the melt fracture during extrusion of a molten narrow molecular weight distribution ethylene polymer by extruding the polymer through a die having a die gap greater than about 50 mils and wherein at least a portion of one surface of the die lip and/or die land in contact with the molten polymer is at an angle of divergenece or convergence relative to the axis of flow of the molten polymer through the die.
U.S. Pat. No. 4,348,349 relates to a process for reducing melt fracture formed during extrusion of a molten narrow molecular weight distribution linear ethylene polymer which comprises extruding said polymer through a die having a discharge outlet defining an exit die gap formed by opposing die lip surfaces and wherein one surface of the die lip and/or die land in contact with the molten polymer extends beyond the opposing surface of the die lip and/or die land in the direction of the axis of flow of the molten polymer through the die exit.
U.S. Pat. No. 4,360,494 describes a process for reducing melt fracture formed during extrusion of a molten narrow molecular weight distribution linear ethylene polymer comprising extruding the polymer through a die having a discharge outlet defining an exit die gap formed by opposing die lip surfaces and wherein one surface of the die lip and/or die land in contact with the molten polymer extends beyond the opposing surface of the die lip and/or die land in the direction of the axis of flow of the molten polymer through the die exit and wherein the extended die lip has a groove extending around the extended die lip. The die groove is disposed opposite the leading edge of the opposing die lip surface.
U.S. Pat. No. 4,415,711 describes a process for forming a blown film from a normally solid thermoplastic resin having an extensional viscosity index of less than about six. The resin is extruded through the die lips of a tubular film die to form a molten tube, the molten tube being expanded radially at an angle of at least 45 degrees.
While these improvements permit the utilization of modified LDPE extrusion equipment for processing LLDPE resin, they do not lead to improvements in film quality or bubble stability. Moreover, they do not permit sensible operation of the same equipment for HDPE, MDPE, LDPE and LLDPE resins.